Chemistry Reference
In-Depth Information
12.3 What Went Wrong?
Besides the very small yield the product showed unexpected properties: a high
reactivity, the behaviour of unsaturated compounds (like adding halogens easily),
the colored appearance, the easy, seemingly reversible peroxidation when brought
into contact with air (oxygen), and worst of all: the elemental analysis resulted in
88 % carbon and 6 % hydrogen, contrasting the theoretical values of 94 % C and
6 % H for (C
6
H
5
)
6
C
2
. Rather than the desired hexaphenylethane Gomberg had
indeed synthesized (among others, facing the tiny yield) a compound that showed
many similarities with what could have been assumed for the so far unknown
triphenylmethyl, (C
6
H
5
)
3
C. The latter could be considered the “monomer” of
(the “dimer”) hexaphenylethane, and had to be considered a
radical
, that is -
with respect to the contemporary theories - a chemical entity with an unsaturated,
trivalent carbon atom. It has to be pointed out, however, that Gomberg did not
isolate this compound in a proper sense.
The concept of a chemical radical was introduced by Antoine Lavoisier (1743-
1794), who attributed it to the assumed crucial parts of acids. J
¨
ns Jacob Berzelius
(1779-1848) used the word in the meaning of an “element imitator” - that is, as a
virtual ordering principle.
9
Chemists have been contributing to the radical research
ever since (cf. the historical surveys in Walden
1924
; Gomberg
1932
; Ihde
1967
;
Rocke
1984
;R¨chardt
1992
; Nye
1993
). In a survey of free radical chemistry
Gomberg himself gave the following historical summary on radicals leading up to
his work on the “perphenylated” ethane (Gomberg
1932
):
1815-1835 Radicals were agencies for classifying the increasing number
of organic substances
1835-1850 It became apparent that the analogy between radicals and elements
was merely formal
1848-1858 The type theory came into being
1860-1900 Valency theory and the quadrivalency doctrine for carbon emerged.
s chemistry - as opposed to the early historical phase featuring Lavoi-
sier and others as well as to the time of Gomberg
In today
'
s original contributions around
1900 - a radical is defined as a particle (atomic or molecular) which contains
unpaired electrons. There is no doubt among modern chemists that (some) radicals
do exist in a substantial, literal sense, although most of the latter are quite unstable
or show only very short lifetimes.
10
For Gomberg and his contemporaries the
'
9
Cf. in the preface of his chemistry textbook Berzelius
1825
.
10
Well-known examples (from modern chemistry) are the nitrogen oxides NO and NO
2
. The most
fascinating exception of the group of instable or intermediate radicals is dioxygen, O
2
. The molecules
of this (normal) allotrope of oxygen contain two unpaired electrons and are reasonably stable.
Accordingly most modernist attempts to set up a historical order for the development of the radical
concept look like the following of R¨chardt
1992
: 1787-1900 Period of the clarification of notion;
1900-1945 Period of discoveries; 1945 - today Period of theoretical achievement and successful
application. In another article the same author even classifies a single electron as radical (R¨chardt
and Mayer-Ruthardt
1969
,41).
